The present invention relates to a data transmission system for transmitting data over telephone lines or other analog lines and, more particularly, to a jitter cancelling apparatus for carrier phase control associated with a receiver of such a data transmission system.
In a data transmission system, signals usually undergo various kinds of deterioration as typified by amplitude distortions, delay distortions, carrier frequency offsets and carrier phase jitters while being transmitted over lines. Among the various kinds of deterioration, amplitude distortions and delay distortions are almost time-invariant or, if time-variant, the variation is slow enough to allow such distortions to be compensated for by so-called automatic equalizers. Carrier phase jitters, on the other hand, result in time-variant distortions which have hitherto been absorbed by a phase locked loop or similar feed-back control system. However, since the carrier phase jitters develop in, for example, a carrier supply device of an analog transmission link and include periodic components approximating the ac (alternate current) cycle of 50 or 60 Hz of a commercial power source, the phase locked loop will fail to absorb the 60 Hz jitters under a baud rate of 2400 Hz unless the phase locked loop equivalent quality factor is limited to less than 40. Limiting the quality factor to less than 40 deteriorates the Gaussian noise suppression ability of the phase locked loop.
According to the prior art, such phase jitters are removed by, for instance, the jitter canceller disclosed in the U.S. Pat. No. 4,639,939. However, since, the tap coefficient of the adaptive filter for use in the prior art jitter canceller is reset to "0" before a data transmission, it cannot cancel phase jitters until the filter coefficient reaches its optimum value. In a usual modem for use from point to point, because of the relatively large amount of data to be transmitted, the duration of no jitter suppression is shorter than the period during which jitters do undergo suppression, and accordingly the data transmission is not substantially affected. However, in a multi-point data transmission system in which a plurality of remote modems are connected to a single line and data are transmitted from these remote modems to a center modem, the amount of the data to be transmitted is so small that data transmission time is completed before the filter converges. As a result, a problem arises in that data are received with jitters totally failing to be suppressed, so that accurate data cannot be received.